It is known to use laser-based and/or imager-based readers in a dual window or bi-optical workstation to electro-optically read indicia, such as bar code symbols, associated with three-dimensional products to be identified and processed, e.g., purchased, at a point-of-transaction workstation provided at a countertop of a checkout stand in supermarkets, warehouse clubs, department stores, and other kinds of retailers. The products are typically slid or moved by a user across, or presented to a central region of, a generally horizontal window that faces upwardly above the countertop and/or a generally vertical or upright window that rises above the countertop. When at least one laser scan line generated by a laser-based reader sweeps over a symbol and/or when return light from the symbol is captured over a field of view by a solid-state imager of an imager-based reader, the symbol is then processed, decoded and read, thereby identifying the product.
The symbol may be located low or high, or right to left, on the product, or anywhere in between, on any of six sides of the product. The symbol may be oriented in a “picket fence” orientation in which elongated parallel bars of a one-dimensional Universal Product Code (UPC) symbol are vertical, or in a “ladder” orientation in which the UPC symbol bars are horizontal, or at any orientation angle in between. The products may be held by the user at various tilt angles during their movement across, or presentation to, either window. The products may be positioned either in contact with, or held at a distance away from, either window during such movement or presentation. All these factors make the symbol location variable and difficult to predict in advance.
In such an environment, it is important that the readers at the workstation provide a full coverage scan zone above the horizontal window and in front of the upright window so that the scan zone extends down as close as possible to the countertop, and sufficiently high above the countertop, and as wide as possible across the width of the countertop. The scan zone projects into space away from the windows and grows in volume rapidly in order to cover symbols on products that are positioned not only on the windows, but also many inches therefrom. The scan zone must be large enough to read symbols positioned in any possible way across the entire volume of the scan zone and must not have any dead areas in which symbols are not covered and, therefore, cannot be read.
As advantageous as workstations with laser-based readers have been in processing transactions, workstations with imager-based readers, also known as imagers, are thought to offer improved reliability and have the added capability of reading indicia other than UPC symbols, such as two-dimensional or stacked or truncated symbols, as well as the capability of imaging non-symbol targets, such as receipts, driver's licenses, signatures, etc. It was initially thought that an all imager-based workstation would require about ten to twelve, or at least six, imagers in order to provide a full coverage scan zone to enable reliable reading of indicia that could be positioned anywhere on all six sides of a three-dimensional product. However, to bring the cost of the imager-based workstation down to an acceptable level, it is known to reduce the need for so many imagers by splitting the field of view of at least one imager into a plurality of subfields of view, each additional subfield serving to replace an additional imager.
However, such subfields of view, also known as light collection regions, produced by splitting the field of view in the known imager-based workstation do not fully occupy the scan zone. As a result, the scan zone does not have full coverage and has dead areas in which indicia cannot be read. Also, at least some of the subfields are twisted or skewed relative to the windows through which they pass. As a result, a peripheral portion of the twisted subfields is clipped and blocked by a workstation wall bounding the window. All these factors, of course, degrade reading performance and efficiency.
In addition, due in part to the differently positioned and differently sized windows, the known imagers are typically provided with imaging lens assemblies that have different optical powers and that are focused at different distances away from the windows. In the prior art, some of the twisted subfields diverge as they pass through the windows, thereby reducing resolution when imaging indicia that is approximately parallel to the windows (a common situation), because the projection of individual sensors on the indicia is also enlarged. If the indicia is being imaged by an outer subfield angled to the right or left of the workstation, then the projection of the sensors on the indicia is stretched to the right or left. If the indicia is being imaged through one window by a central subfield directed towards the opposite window, then the sensor projection is stretched at right angles to the stretch from the outer subfields. In other words, for each window, there are subfields that have a higher resolution along a left/right axis, and different subfields have a higher resolution along a perpendicular up/down axis.
Anamorphic optics in the prior art served to squash the subfield in the direction where the angled projection of the subfield through the windows would otherwise have stretched the subfield and the sensors, thereby modifying the resolution to the point where high density indicia could not be adequately resolved. Elimination of anamorphic optics in the imaging lens assemblies would reduce the cost of the workstation and increase manufacturing efficiency.
Also, the splitting of the field of view in the known imager-based workstation is performed by positioning an optical splitter very close to the imager. Since each subfield grows as the distance from the imager is increased, the positioning accuracy of a closely adjacent splitter is very critical in the prior art, thereby adding a labor-intensive and costly additional adjustment step to the manufacture process. In addition, since the imagers are focused at a distance (typically a few inches) beyond the respective window, the image of any indicia close to the imager is very blurred. When the optical splitter is positioned very close to the imager, the edges of the optical splitter are very blurred and unfocussed, and there is not a sharp delineation between the subfields being split. Since each split subfield is very small in spatial volume, this blur can actually cover a significant portion of each split subfield, thereby wasting precious sensors and reducing the number of sensors available to resolve indicia located in the scan zone.